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Catalytic reforming involves converting low octane hydrocarbons, such as hexane, into more valuable high octane reformates, such as benzene. Catalytic reforming produces hydrogen as a by-product (1). Low octane hydrocarbons are typically found in naphtha. Naphtha is the name given to the distillation product formed during petroleum refining and along with reformate they are "complex mixtures of parafï¬ns, naphthenes, and aromatics in the C5 -C12 range" (2). Certain naphthas have also been found to contain elements which are known to harm platinum and rhenium catalysts used in catalytic reforming. These elements, including sulphur and nitrogen, must first be removed from the naphtha before carrying out the reforming process to avoid damage to the catalyst (2). Composition of different naphthas varies depending on the origin of the crude oil from which the naphtha was formed (7).
High octane products are not naturally occurring and are not produced by simple crude oil distillation so need to be produced by the process of catalytic reforming. These high octane products are much more useful in industry and are useful to increase the octane number of petrol. Increasing the octane number of petrol reduces risk of damage to the engine it is used in by as it allows the petrol to become more resistive to knocking. There are various different reactions which involve reforming. Some of the most important chemical reactions during catalytic reforming are shown below (fig. 1). (3), (4), (6)
Figure 1. Key reactions in catalytic reforming of naphtha (2).
Most of the reactions shown above produce hydrogen as a by-product, which has become quite useful to remove sulphur and nitrogen from naphtha mixtures (2). According to Ali (2006), this makes reactions (a), (b) and (c) the most desirable catalytic reforming reactions and that these three reactions are favoured thermodynamically by high temperature and low pressure. Also from (fig. 1) it can be seen that hydrocracking and hydrogenolysis processes make use of hydrogen so are therefore the least desirable forms of catalytic reforming reactions. Coke formation is also undesired as this process will ultimately deactivate the catalyst. Figure 4 overleaf gives shows a processing scheme for refinery petrol production including catalytic reforming.
The production of benzene from hexane, see (fig. 2), is one of the most important industrial catalytic reforming processes and is widely used to make "solvent, synthetic rubber, dye, and drug manufacture" (3), (4).
Figure 2. Reforming reaction of hexane to benzene (1).
Toluene is another high octane product of catalytic reforming, which is formed by the reforming of heptane, see (fig. 3) (1). Toluene is an important product which can be used to increase the octane rating of petrol (3), (4).
Figure 3. Reaction to show production of toluene from heptane (1).
According to Antos and Aitani (2004), catalytic reforming is "carried out at an elevated temperature (450-500oC) and moderate pressure (4-30 bar)". This means that any catalyst used would have to be able to withstand this high temperature without being deactivated. Catalytic reforming processes make use of noble metal catalysts with the main two being platinum and rhenium. Often these two catalysts are used together mixed with an alumina (aluminium oxide) molecule (bifunctional catalyst) (2).
In industry two main combinations of bifunctional catalysts are used, the first being a mixture of platinum, rhenium and alumina and the second a mixture of platinum, tin and alumina. Pt-Re/Al2O3 is the more robust of the two and is therefore more favourable in semi regenerative reforming where a catalyst should be able to last about 1-2 years before regeneration as this involves shutting down reactors periodically to allow for catalyst regenration. Pt-Sn/Al2O3 is more favourable in continuous regeneration processes as it offers higher selectivity at lower pressure, and as it is in a continuous process, will be regenerated every 6-8 days according to Antos and Aitani (2004).
Figure 4. Example of a processing scheme for refinery petrol production with catalytic reforming (2).
As the main catalytic reforming processes operate at high temperatures the cost for operating at such a high temperature will be high so a trade-off must be made to give a reasonable product yield and operating cost. As optimum pressure is relatively low operating costs relating to pressure would not be too high.
In terms of catalyst the main element involved is platinum and the price of platinum is currently around $2000 an ounce which is around $57 per Kg (5). However according to Antos and Aitani, the following three main things must be done to achieve optimal performance and cost:
Use a low content of platinum, typically <0.5 wt%, with "maximum atomic dispersion".
Acid sites must be in close immediacy of particles of platinum.
"Adequate mechanical properties".
However a patent by Sinfelt (1975), has suggested that adding a third miscible metal such as nickel to a mixture of two immiscible metals such as platinum and rhenium can substantially improve the functionality of the catalyst and could result in greater profitability (8). This type of catalyst would be known as a multimetallic catalyst. Antos and Aitani suggest that the use of multimetallic catalysts will be a revelation to the reforming industry due to the fact that they will offer both improved "selectivity and activity".
On the other hand, as the technology of multimetallic catalysts has been around for a while, one must wonder as to why they are not widely used in the reforming industry. This again comes down to the cost of producing multimetallic combinations. It must be considered if the extra cost of creating the catalyst will offer enough improvement to make it a profitable industrial option.
Current Research in Catalytic Reforming
The main advances in catalytic reforming have been fuelled by new strict government legislation, a desire to improve octane ratings and yield of reformate. Also in recent years hydrogen produced from reforming processes has become very important due to the increased investment in hydrogen fuel technology. This has lead to improved catalyst activity being needed which has come from using multimetallic technology. The main catalyst used in industry for continuous reforming processes is now Pt-Sn/Al2O3 when originally only platinum catalysts were used, which then moved to a bimetallic catalyst of platinum and rhenium being used. The advantage of using Pt-Sn/Al2O3 is that it offers increased activity and selectivity and a lower rate of deactivation as compared to a platinum only catalyst (2).
More recent research is also being done to find a cheaper alternative to using expensive metals such as platinum as catalysts. Chettapongsaphan, et al. (2010) researched the use of a less expensive reforming catalyst La0.8Sr0.2Cr0.9Ni0.1O3 which gave similar reforming activity as the more expensive metallic based catalysts. Also this catalyst also is shown to have greater resistance to deactivation by coking (9).
Another use of reforming currently is using the reforming process to use the hydrogen by-product formed to produce hydrogen fuel cells. A recent article uses the autothermal reforming of biodiesel to produce hydrogen rich gas which can be used in fuel cells (10).
Recent research into platinum catalysts has also found that coating a single crystal of platinum with carbon monoxide can form "nanoclusters" of platinum which offer greater stability than single crystals (5), and could potentially improve the performance of the catalyst and reduce the cost as less platinum may be needed due to carbon monoxide being attached to it.
A further innovation in catalytic reforming is the development of the RZ Platforming process by UOP. The process is a fixed bed system and is a more efficient way to convert paraffins and naphthenes to aromatics (11). Figure 5 overleaf gives an overview of the RZ Platforming process.
Figure 5. Overview of RZ Platforming process (11).
The benefits of the RZ Platforming process are that:
Offers higher hydrogen yields compared to conventional reforming processes.
Offers constant aromatic selectivity of about 80% (11).
Figure 6 below shows a graphical representation of the higher hydrogen yield offered by the RZ Platforming process as compared to a conventional reforming process.
Figure 6. Comparison of hydrogen yields of RZ Platforming and CCR Platforming (11).